Hyperspectral imaging (HSI) technology takes advantage of the wavelength composition of electromagnetic radiation (EMR). Almost all electromagnetic radiation, including visible light, is comprised of a mixture of many different wavelengths of EMR. Electromagnetic waves embody the physical interaction between an electric field and a magnetic field that creates the wave. Electromagnetic waves can be in the form of visible light but can also include X-rays, gamma rays, microwaves, radio waves, and other spectra of electromagnetic radiation. These forms of electromagnetic radiation are distinguished by their wavelengths.
Wavelengths of interest for HSI can range from the low end of the visible spectrum (violet) through the high end of the visible spectrum (red), through near-infrared (NIR), short-wave infrared (SWIR) and mid-wave infrared (MWIR) to long-wave infrared (LWIR). Long-wave infrared is also called thermal infrared. Most HSI systems utilize only part of this wavelength range of interest.
When electromagnetic radiation strikes an object such as a solid, a liquid, or a gas, the electromagnetic radiation can be reflected, absorbed, transmitted, or some combination of two or more of these processes. When EMR is reflected, the EMR is turned back from the surface of the object (substance). When EMR is absorbed, the EMR is received and taken in by the substance. When EMR is transmitted, the EMR passes through the substance. For example, when light (visible EMR) strikes a body of water, some of the light is reflected. An observer can see their reflection in the water. Some of the light striking the water is absorbed. The energy of the light warms the water and decreases the intensity of the light as it travels through the water. Finally, some of the light is transmitted through the water, enabling the observer to see the bottom of the pool. The physical characteristics of the substance (that is, the water) and the wavelength of the EMR determine how much of the EMR is reflected, how much of the EMR is absorbed, and how much of the EMR is transmitted.
EMR often includes a mixture of electromagnetic radiation of different wavelengths. A prism can be used to separate EMR that is comprised of a mixture of different wavelengths into its constituent wavelengths. Likewise, a diffraction grating can also be used to disperse EMR into its spectrum. A diffraction grating is often a material with a reflective surface onto which thousands of very fine, parallel grooves have been etched. The grooves are etched at specific angles, and high-precision diffraction gratings can have over a thousand grooves per millimeter on their surface. The grooves reflect incident EMR into its spectrum. Diffraction gratings provide more accurate and more consistent spectral dispersion than prisms. The wavelengths that are dispersed by the grating can be controlled by the angle, width, and spacing of the grooves of the diffraction grating. In a hyperspectral sensor, EMR is captured and focused onto a diffraction grating, and the diffraction grating spreads the incident electromagnetic radiation into its constituent wavelengths. By using a diffraction grating, the wavelength spread is predictable and very accurate.
After an EMR spectrum is dispersed, it can be divided into separate bands, and the intensity of the EMR in each of the bands can be measured. By spreading the EMR into its spectrum and then measuring the intensity of its different wavelengths, a user can differentiate between different EMR samples that would otherwise appear to be the same. The process of measuring the intensity of wavelengths in an EMR sample provides the ability to detect specific combinations of wavelengths. That is, the spectrum of EMR reflected by an object is used to detect and determine the composition of the object. The spectrum of EMR reflected by an object depends upon the EMR used to illuminate the object and the reflectance of the object. Each object's reflectance can be plotted as a function of wavelength to provide a spectral signature of the object. A hyperspectral system can then detect objects by comparing the measured reflected EMR with a library of spectral signatures. When a match is found, the object is identified.
The EMR that reflects from the diffraction grating is directed to a set of collectors that converts the incident light of various wavelengths into electrical signals. The collectors can be contained on an electronic chip that is very much like the electronic chips that are found in digital cameras, except the chips in an HSI collector (sensor) are usually sensitive to much wider ranges of EMR while a digital camera is sensitive only to visible light. The electrical signals from the chip are passed to a computer that processes the data and presents the user with information that can be used to reach conclusions about the source of the captured EMR, that is, the identity of the object reflecting the EMR.
HSI sensors receive the EMR and convert it to electrical signals. HSI sensors use lenses that focus the incoming EMR, a slit that limits the incoming EMR to a thin (but wide) beam, a diffraction grating that disperses the thin wide beam into its spectra, and photo-receptors that collect the EMR in specific wavelength bands and convert the band intensities to electrical signals.
Spectra can be collected over an entire area encompassing the sample object simultaneously using an electronically tunable optical imaging filter such as an acousto-optic tunable filter or a liquid crystal tunable filter. The materials in optical filters produce the desired bandpass and transmission function. The spectra obtained for each pixel of the image forms a complex data set that is the hyperspectral image. The hyperspectral image can contain intensity values at numerous wavelengths.
In hyperspectral imaging systems, radiation reflected by or emanating from a target or specimen is detected in a large number of narrow contiguous spectral bands, producing a data set which is distributed not only spatially, but spectrally as well. That is, for each pixel within an image of the target, information is recorded in each of the spectral bands, thereby producing a three-dimensional hyperspectral image cube, a 3 angle dimension field, in which spectral information for each pixel is distributed across a spectral axis perpendicular to the spatial axes.
Image sensors capture image content at multiple wavelengths. The resulting data is formatted electronically as a data cube or image cube that includes stacked two-dimensional layers of images. Each layer corresponds to a particular wavelength of the imaged surface. The multiple wavelength images of the same physical location are arranged on top of each other to form the stack.
Hyperspectral imaging systems of this sort are often very complex, expensive, and large. They often require complex calibration and compensation to account for changing ambient illumination conditions, including conditions related to the illuminating EMR, atmospheric scattering, atmospheric absorption, the reflected EMR from the object, and the EMR that reaches the sensor. To correct for these shortcomings, HSI systems are often large, expensive and unwieldy devices that are unsuitable for active applications. These systems often have inherent design limitations related to motion of the associated transmission and reception platforms, motion or changes in the atmosphere, and/or motion of the objects in the image field that occur during scan sequences. These effects can lead to reduced resolution and ineffective observations. HSI systems include focal plane arrays that are expensive and difficult to create for large frequency bands and for low frequencies.
In view of the above, there is a need for an improved imaging system and method that overcomes the problems or disadvantages in the prior art.